Magnetic sensing device with reduced shield-to-shield spacing

ABSTRACT

A magnetic sensor assembly includes first and second shields each comprised of a magnetic material. The first and second shields define a physical shield-to-shield spacing. A sensor stack is disposed between the first and second shields and includes a seed layer adjacent the first shield, a cap :layer adjacent the second shield, and a magnetic sensor between the seed layer and the cap layer. At least a portion of the seed layer and/or the cap layer comprises a magnetic material to provide an effective shield-to-shield spacing of the magnetic sensor assembly that is less than the physical shield-to-shield spacing.

BACKGROUND

In an electronic data storage and retrieval system, a magnetic recordinghead typically includes a reader portion having a sensor for retrievingmagnetically encoded information stored on a magnetic disc. Magneticflux from the surface of the disc causes rotation of the magnetizationvector of a sensing layer or layers of the sensor, which in turn causesa change in the electrical properties of the sensor. The sensing layersare often called free layers, since the magnetization vectors of thesensing layers are free to rotate in response to external magnetic flux.The change in the electrical properties of the sensor may be detected bypassing a current through the sensor and measuring a voltage across thesensor. Depending on the geometry of the device, the sense current mayhe passed in the plane (CIP) of the layers of the device orperpendicular to the plane (CPP) of the layers of the device. Externalcircuitry then converts the voltage information into an appropriateformat and manipulates that information as necessary to recoverinformation encoded on the disc.

Contemporary read heads typically include a thin film multilayerstructure containing ferromagnetic material that exhibits some type ofmagnetoresistance (MR). A typical MR sensor configuration includes amultilayered structure formed of a nonmagnetic layer positioned betweena synthetic antiferromagnet (SAF) and a ferromagnetic free layer, orbetween two ferromagnetic free layers. The resistance of the MR sensordepends on the relative orientations of the magnetization of themagnetic layers.

An MR sensor may include shields consisting of high permeabilitymaterials that function to protect the sensor from stray magnetic fieldsoriginating from adjacent magnetic bits on the medium. With decreasingsensor size, the shield-to-shield spacing of the MR sensor should bemade smaller to adequately screen the flux from adjacent bits. However,the seed and cap layers of the magnetic sensor occupy a large proportionof the total stack thickness and provide a limit to the reduction of theshield-to-shield spacing for magnetic sensors.

SUMMARY

The present invention relates to a magnetic sensor including first andsecond shields each comprised of a magnetic material. The first andsecond shields define a physical shield-to-shield spacing. A sensorstack is disposed between the first and second shields and includes aseed layer adjacent the first shield, a cap layer adjacent the secondshield, and a magnetically sensitive portion between the seed layer andthe cap layer. At least a portion of the seed layer and/or the cap layercomprises a magnetic material to provide an effective shield-to-shieldspacing of the magnetic sensor assembly that is less than the physicalshield-to-shield spacing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a layer diagram of a magnetic sensor assembly includingmagnetic first shield, seed, and second shield layers.

FIG. 2 is a graph showing the pinning field of the magnetic sensorassembly of FIG. 1 as a function of the thickness of the magnetic seedlayer.

FIG. 3 is a layer diagram of a magnetic sensor assembly includingmagnetic first shield layer, a magnetic seed layer, partially magneticcap bilayer, and a magnetic second shield layer.

FIG. 4 is a layer diagram of a magnetic sensor assembly includingmagnetic first shield layer, a nonmagnetic seed layer, a magnetic caplayer, and a magnetic second shield layer.

FIG. 5 is a layer diagram of a magnetic sensor assembly includingmagnetic first shield layer, a magnetic/nonmagnetic seed bilayer, and amagnetic second shield layer.

DETAILED DESCRIPTION

FIG. 1 is a layer diagram of magnetic sensor assembly 10 including firstshield layer 12, seed layer 14, pinning layer 16, pinned layer 18,coupling layer 20, reference layer 22, barrier layer 24, first freelayer 26, second free layer 28, cap layer 30, and second shield layer32. First shield layer 12 and second shield layer 32 are made of amagnetic material, such as NiFe, NiFeCu, or NiCoFe. Sensor stack 36includes seed layer 14, cap layer 30, and magnetic sensor 38. Magneticsensor 38 includes pinning layer 16, pinned layer 18, coupling layer 20,reference layer 22, barrier layer 24, first free layer 26, and secondfree layer 28. Sensor stack 36 may be formed by depositing successivelayers on first shield layer 12 or may be formed in a separate processand later incorporated between first shield layer 12 and second shieldlayer 32.

The magnetization of reference layer 22 is fixed while themagnetizations of free layers 26 and 28 rotate in response to anexternal magnetic field from a magnetic medium. Pinned layer 18 andreference layer 22 are magnetically coupled by coupling layer 20 andmake up a synthetic antiferromagnet (SAF). The magnetization directionof pinned layer 18, which is opposite the magnetization direction ofreference layer 22, is pinned by exchange coupling pinning layer 16 withpinned layer 18. Seed layer 14 enhances the grain growth of pinninglayer 18 and cap layer 30 adds a protective layer to the top of magneticsensor 18. First shield 12 and second shield 32 protect magnetic sensor38 from flux emanated from adjacent tracks on the magnetic medium.

Magnetic sensor assembly 10 produces a signal when a sense current ispassed through the layers of sensor stack 36. In some embodiments, firstshield 12 and second shield 32 deliver the sense current to sensor stack36. The sense current experiences a resistance that is proportional tothe angle between the magnetization direction of free layers 26 and 28and the magnetization direction of reference layer 22.

While a magnetic sensor 38 is shown as a TMR sensor in FIG. 1, it willbe appreciated that other sensor configurations are possible. Forexample, magnetic sensor 38 may alternatively be configured to exhibit agiant magnetoresistance (GMR) or other types of magnetoresistance. Inaddition, pinning layer 16, pinned layer 18, coupling layer 20, andreference layer 22 may alternatively be replaced by one or more freelayers to provide a tri-layer type magnetic sensor.

Magnetic sensor assembly 10 has a physical shield-to-shield spacingss_(p). The linear density of magnetic sensor assembly 10 is stronglycorrelated to the shield-to-shield spacing of magnetic sensor assembly.Consequently, in order to increase the linear density (i.e., the numberof magnetic transitions per unit length) of magnetic sensor assembly 10,the shield-to-shield spacing may be reduced from physicalshield-to-shield spacing ss_(p). One approach to accomplishing this isto form seed layer 14, which has a thickness t_(seed), of a magneticmaterial. The material selected for seed layer 14 may have some or allof the following properties: magnetically soft, crystalline structurethat matches the crystalline structure of first shield layer 12,adequate pinning field for pinned layer 18 of magnetic sensor 38, lowelectrical resistance, high corrosion resistance, and high bulkoxidation resistance. Alternatively, seed layer 14 may be made of anamorphous material. In some embodiments, seed layer 14 has a coercivityof less than about 2.0 Oe. In other embodiments, seed layer 14 has acoercivity of less than about 10 Oe. In further embodiments, seed layer14 has a coercivity of more than about 10 Oe. In addition, themagnetostriction and/or magnetic anisotropy of the material may beconsidered in selecting the material for seed layer 14. In someembodiments, seed layer 14 is comprised of NiFe, NiFeNb, NiFeTa, NiFeRh,CoZrTa, CoZrNb, CoZrNd, CoFeB, CoFeTa, CoFeZr, CoFeRh, CoFe, CoCr, orNi_(>70%)Cr_(<30%). In the case of CoZrTa, the atomic percentage of eachof Ta and Zr may be in the range of about 2% to about 10%. When seedlayer 14 is made of a material having these properties, theshield-to-shield spacing of magnetic sensor assembly 10 is reduced fromphysical shield-to-shield spacing ss_(p) by the thickness t_(seed) ofseed layer 14 to an effective shield-to-shield spacing ss_(eff). Inother words, effective shield-to-shield spacing ss_(eff) is defined bythe distance between magnetic layers most proximate to sensor stack 38in magnetic sensor assembly 10. In some embodiments, seed layer 14 has athickness t_(seed) between about 10 Å and about 1,000 Å. In analternative embodiment, the physical shield-to-shield spacing ss_(p) maybe reduced by eliminating seed layer 14 entirely.

Devices substantially similar to magnetic sensor assembly 10 werefabricated and tested to determine their resistance-area (RA) product ofthe devices and the magnetoresistive (MR) ratio. The MR ratio is thechange in resistance exhibited by the device in response to changes inthe sensed external field divided by the total resistance across thedevice. The devices fabricated included a 50 Å seed layer 14 comprisingNiFe, a 70 Å pinning layer 16 comprising IrMn, an 18 Å pinned layer 18comprising CoFeB, a 9 Å coupling layer 20 comprising Ru, an 20 Åreference layer 22 comprising CoFeB, an oxide barrier layer 24 havingvarying thicknesses, a 15 Å first free layer 26 comprising CoFe, a 30 Åsecond free layer 28 comprising NiFe, and a 175 Å cap layer 30comprising Ta. The following table shows the ranges of RA product and MRratio values and the median RA product and MR ratio measured for variousthicknesses for barrier layer 24.

Barrier Median Layer RA Product Median RA MR Ratio MR Thickess RangeProduct Range Ratio (Å) (Ω-μm²) (Ω-μm²) (%) (%) 3.6 1.254-2.219 1.70016.91-22.36 20.25 4.0 1.978-3.544 2.749 17.59-22.67 20.97 4.43.499-5.286 4.712 18.82-23.16 21.75 4.8 7.040-11.19 8.663 18.78-22.7321.83

The magnetic stability of magnetic sensor 38 can also be improved bysetting the thickness t_(seed) of seed layer 14 to provide a largepinning field. FIG. 2 is a graph showing the pinning field H_(p) as afunction of thickness t_(seed) of seed layer 14. As can be seen, as thethickness t_(seed) of seed layer 14 is increased, the pinning field inmagnetic sensor assembly 10 also increases. Since seed layer 14 made ofa magnetic material does not affect the effective shield- to-shieldspacing ss_(eff) of magnetic sensor assembly 10, seed layer 14 can bemade sufficiently thick to maximize the pinning field to provideincreased device stability.

Variations on the design shown in FIG. 1 may be made to adjust orfurther reduce the effective shield-to-shield spacing ss_(eff) of themagnetic sensor assembly. For example, FIG. 3 is a layer diagram of amagnetic sensor assembly 40 including first shield layer 42, seed layer44, pinning layer 46, pinned layer 48, coupling layer 50, referencelayer 52, barrier layer 54, first free layer 56, second free layer 58,first cap layer 60, second cap layer 62, and second shield layer 64.First shield layer 42 and second shield layer 64 are made of a magneticmaterial, such as NiFe, NiFeCu, or NiCoFe. Sensor stack 66 includes seedlayer 44, pinning layer 46, pinned layer 48, coupling layer 50,reference layer 52, barrier layer 54, first free layer 56, second freelayer 58, first cap layer 60, and second cap layer 62. Magnetic sensor68 includes pinning layer 46, pinned layer 48, coupling layer 50,reference layer 52, barrier layer 54, first free layer 56, and secondfree layer 58. Sensor stack 66 may be formed by depositing successivelayers on first shield layer 42 or may be formed in a separate processand later incorporated between first shield layer 42 and second shieldlayer 64.

The operation of magnetic sensor assembly 40 is substantially similar tothe operation of magnetic sensor assembly 10 as described with regard toFIG. 1. In this embodiment, seed layer 44 is again made of a magneticmaterial, and the cap assembly is a bilayer structure including firstcap layer 60 made of a conventional capping material (e.g., Ta) andsecond cap layer 62 made of a magnetic material. The magnetic materialselected for seed layer 44 and second cap layer 62 may have some or allof the following properties: magnetically soft, crystalline structurethat matches the crystalline structure of the adjacent shield layerand/or pinning layer 46 (to promote crystalline texture, for example),adequate pinning field for pinned layer 48 of magnetic sensor 68, lowelectrical resistance, high corrosion resistance, and high bulkoxidation resistance. Alternatively, seed layer 44 and/or second caplayer 62 may be made of an amorphous material (to break texture betweenadjacent layers, for example). In some embodiments, seed layer 44 has acoercivity of less than about 2.0 Oe. In other embodiments, seed layer44 has a coercivity of less than about 10 Oe. In further embodiments,seed layer 44 has a coercivity of more than about 10 Oe. In addition,the magnetostriction and/or magnetic anisotropy of the material may beconsidered in selecting the material for seed layer 44 and second caplayer 62. In some embodiments, seed layer 44 and second cap layer 62 arecomprised of NiFe, NiFeNb, NiFeTa, NiFeRh, CoZrTa, CoZrNb, CoZrNd,CoFeB, CoFeTa, CoFeZr, CoFeRh, CoFe, CoCr, or Ni_(>70%)Cr_(>30%). Whenseed layer 44 and second cap layer 62 are each made of a material havingthese properties, the shield-to-shield spacing of magnetic sensorassembly 40 is reduced from physical shield-to-shield spacing ss_(p) bythe thickness t_(seed) of seed layer 44 and the thickness of t_(cap) ofsecond cap layer 62 to an effective shield-to-shield spacing ss_(eff).In other words, effective shield-to-shield spacing ss_(eff) is definedby the distance between magnetic layers most proximate to sensor stack68 in magnetic sensor assembly 40. In some embodiments, seed layer 44and second cap layer 62 each has a thickness t_(seed) between about 10 Åand about 1,000 Å. In an alternative embodiment, a single magnetic caplayer is formed on magnetic sensor 68 instead of the bilayer capassembly shown.

FIG. 4 is a layer diagram of magnetic sensor assembly 70 including firstshield layer 72, seed layer 74, pinning layer 76, pinned layer 78,coupling layer 80, reference layer 82, barrier layer 84, first freelayer 86, second free layer 88, nonmagnetic cap layer 90, magnetic caplayer 92, and second shield layer 94. First shield layer 72 and secondshield layer 94 are made of a magnetic material, such as NiFe, NiFeCu,or NiCoFe. Sensor stack 96 includes seed layer 74, pinning layer 76,pinned layer 78, coupling layer 80, reference layer 82, barrier layer84, first free layer 86, second free layer 88, and magnetic cap layer92. Magnetic sensor 98 includes pinning layer 76, pinned layer 78,coupling layer 80, reference layer 82, barrier layer 84, first freelayer 86, and second free layer 88. Sensor stack 96 may be formed bydepositing successive layers on first shield layer 72 or may be formedin a separate process and later incorporated between first shield layer72 and second shield layer 94.

The operation of magnetic sensor assembly 70 is substantially similar tothe operation of magnetic sensor assembly 10 as described with regard toFIG. 1. In this embodiment, seed layer 74 is made of a nonmagneticmaterial (e.g., Ru or Cu), cap layer 90 is made of a nonmagneticmaterial, and cap layer 92 is made of a magnetic material. The magneticmaterial selected for cap layer 92 may have some or all of theproperties described above with regard to cap layer 62. Seed layer 74comprised of a nonmagnetic material, which decouples pinning layer 76from first shield layer 72, can be made very thin to minimize thecontribution to the shield-to-shield spacing. Consequently, the physicalshield-to-shield spacing ss_(p) of magnetic sensor 70 is reducedcompared to similar devices including a seed layer made of conventionalmaterials. In addition, magnetic cap layer 92 reduces theshield-to-shield spacing of magnetic sensor assembly 70 from physicalshield-to-shield spacing ss_(p) by the thickness t_(cap) of magnetic caplayer 92 to an effective shield-to-shield spacing ss_(eff). In someembodiments, magnetic cap layer 92 each has a thickness t_(cap) betweenabout 10 Å and about 1,000 Å.

FIG. 5 is a layer diagram of magnetic sensor assembly 100 includingfirst shield layer 102, first seed layer 104, second seed layer 106,pinning layer 108, pinned layer 1110, coupling layer 1.12, referencelayer 114, barrier layer 116, first free layer 118, second free layer120, cap layer 122, and second shield layer 124. First shield layer 102and second shield layer 124 are made of a magnetic material, such asNiFe, NiFeCu, or NiCoFe. Sensor stack 126 includes first seed layer 104,second seed layer 106, pinning layer 108, pinned layer 110, couplinglayer 112, reference layer 114, barrier layer 116, first free layer 118,second free layer 120, and cap layer 122. Magnetic sensor 128 includespinning layer 108, pinned layer 110, coupling layer 112, reference layer114, barrier layer 116, first free layer 1118, and second free layer120. Sensor stack 126 may be formed by depositing successive layers onfirst shield layer 102 or may be formed in a separate process and laterincorporated between first shield layer 102 and second shield layer 124.

The operation of magnetic sensor assembly 100 is substantially similarto the operation of magnetic sensor assembly 10 as described with regardto FIG. 1. In this embodiment, the seed assembly is a bilayer structureincluding first seed layer 104 made of a magnetic material and secondseed layer 106 made of a nonmagnetic material. The magnetic materialselected for first seed layer 104 may have some or all of the propertiesdescribed above with regard to seed layer 114 or 44. Second seed layer106 is comprised of a nonmagnetic material, such as Ru or Cu, todecouple pinning layer 108 from first shield layer 102. Second seedlayer 106 is very thin to minimize its contribution to theshield-to-shield spacing of magnetic sensor 100, while maximizing theblocking temperature or magnetic sensor 100 and the pinning field frompinning layer 108. Consequently, the physical shield-to-shield spacingss_(p) of magnetic sensor 100 is reduced. In addition, when first seedlayer 104 is made of a magnetic material, the shield-to-shield spacingof magnetic sensor assembly 100 is reduced from physicalshield-to-shield spacing ss_(p) by the thickness t_(seed) of first seedlayer 104 to an effective shield-to-shield spacing ss_(eff). In someembodiments, first seed layer 104 each has a thickness t_(seed) betweenabout 10 Å and about 1,000 Å. In an alternative embodiment, cap layer122 is also made of a magnetic material.

In an exemplary embodiment of magnetic sensor assembly 100, first seedlayer 104 is made of CoZrTa (CZT) and second seed layer 106 is made ofRu. CZT is an amorphous magnetic material with magnetization that can bematched to the adjacent magnetic first shield layer 102, The Ru ofsecond seed layer 106 may be as thin as 15 Å and still provide propertexture to grow subsequent layers of magnetic sensor assembly 100. Insome embodiments, the Ta and Zr of the CZT first seed layer 104 may eachhave atomic percentages in the range of about 2% to about 10%.

The magnetoresistive ratio (ΔR/R) in a magnetic sensor assembly 100including a CZT/Ru bilayer is substantially similar to a correspondingmagnetic sensor assembly including a conventional Ta/Ru seed assembly.In addition, the blocking temperature of magnetic sensor assembly 100including a CZT/Ru bilayer is 275° C., compared to a blockingtemperature of 270° C. for a Ta/Ru seed assembly. Furthermore, thepinning field of magnetic sensor assembly 100 including a CZT/Ru bilayeris greater than a corresponding magnetic sensor assembly including aTa/Ru seed assembly, as shown in the following table.

Pinning Field Seed Layer (Oe) Ta (30 Å)/Ru (30 Å) 624.5 Ta (30 Å)/Ru (15Å) 603.0 CoZrTa (30 Å)/Ru (30 Å) 652.5 CoZrTa (30 Å)/Ru (15 Å) 635.9Consequently, magnetic sensor 100 including a CZT/Ru seed assemblyprovides substantially similar or superior performance to a conventionalTa/Ru seed assembly while reducing the effective shield-to-shieldspacing ss_(eff) by the thickness of the CZT layer.

In summary, the present invention relates to a magnetic sensor includingfirst and second shields each comprised of a magnetic material. Thefirst and second shields define a physical shield-to-shield spacing. Asensor stack is disposed between the first and second shields andincludes a seed layer adjacent the first shield, a cap layer adjacentthe second shield, and a magnetic sensor between the seed layer and thecap layer. At least a portion of the seed layer and/or the cap layercomprises a magnetic material to provide an effective shield-to-shieldspacing of the magnetic sensor assembly that is less than the physicalshield-to-shield spacing. By reducing the shield-to-shield spacing ofthe magnetic sensor, the linear density of the magnetic sensor isincreased. This allows for reading of higher density media and improvesshielding of the magnetic sensor from flux from adjacent bits. Inaddition, a magnetic seed layer of sufficient thickness results in alarge pinning field, thereby improving the magnetic stability of thedevice.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, while the magnetic sensorsaccording to the present invention have been described in the context ofmagnetic recording head applications, it will be appreciated that themagnetic sensors may be used in a variety of other applications,including magnetic random access memory applications.

1-20. (canceled)
 21. A method for fabricating a magnetic sensorassembly, the method comprising: forming a bottom shield; forming afirst seed layer comprising a magnetic material above the bottom shield;forming a sensor stack above the first seed layer, wherein the sensorstack includes a magnetic sensor; forming a first cap layer above thesensor stack; and forming a top shield above the first cap layer,wherein the first seed layer, the sensor stack, and the first cap layercontribute to a physical shield-to-shield spacing of the magnetic sensorassembly, and wherein the first seed layer does not contribute to aneffective shield-to-shield spacing of the magnetic sensor assembly. 22.The method of claim 21, wherein the first seed layer comprises amagnetic material selected from the group consisting of NiFeNb, NiFeTa,NiFeRh, CoZrTa, CoZrNb, CoZrNd, CoFeB, CoFeTa, CoFeZr, CoFeRh, CoFe,CoCr, or Ni_(>70%)Cr_(<30%).
 23. The method of claim 21, wherein thefirst seed layer comprises an amorphous magnetic material.
 24. Themethod of claim 21, wherein the first seed layer comprises a magneticmaterial having a coercivity of less than about 10.0 Oe.
 25. The methodof claim 21, wherein the first seed layer has a thickness of betweenabout 10 Å and about 1,000 Å.
 26. The method of claim 21, furthercomprising: forming a second seed layer between the bottom shield andthe first seed layer, wherein the second seed layer comprises anonmagnetic material.
 27. The method of claim 21, wherein the magneticsensor comprises a tunneling magnetoresistive (TMR) sensor.
 28. Themethod of claim 21, wherein the first cap layer comprises a magneticmaterial, wherein the first cap layer does not contribute to theeffective shield-to-shield spacing of the magnetic sensor assembly. 29.The method of claim 28, further comprising: forming a second cap layerbetween the top shield and the first cap layer, wherein the second caplayer comprises a nonmagnetic material
 30. The method of claim 21,wherein a crystalline structure of the bottom shield matches acrystalline structure of the first seed layer.
 31. The method of claim21, wherein the forming the first seed layer steps includes directlydepositing the magnetic material on the bottom shield.
 32. A method forfabricating a magnetic sensor assembly, the method comprising:depositing a first seed layer comprising a magnetic material on a bottomshield; depositing a sensor stack on the first seed layer; depositing afirst cap layer on the sensor stack; and depositing a top shield on thefirst cap layer, wherein the first seed layer, the sensor stack, and thefirst cap layer contribute to a physical shield-to-shield spacing of themagnetic sensor assembly, and wherein the first seed layer does notcontribute to an effective shield-to- shield spacing of the magneticsensor assembly.
 33. The method of claim 32, wherein the first seedlayer comprises a magnetic material selected from the group consistingof NiFeNb, NiFeTa, NiFeRh, CoZrTa, CoZrNb, CoZrNd, CoFeB, CoFeTa,CoFeZr, CoFeRh, CoFe, CoCr, or Ni_(>70%)Cr_(<30%).
 34. The method ofclaim 32, wherein the first seed layer comprises an amorphous magneticmaterial.
 35. The method of claim 32, wherein the first seed layercomprises a magnetic material having a coercivity of less than about10.0 Oe.
 36. The method of claim 32, wherein the first seed layer has athickness of between about 10 Å and about 1,000 Å.